To properly operate an electro-optic (EO) modulator, minimum RF reflection at the input port (in most cases it is the input connector) is one of the most important specifications since the RF reflection could destabilize the operation of the RF signal source if the RF reflection level is too high, or corrupt the signal fidelity by adding out of phase reflected artifacts to the signal path. There are many sources of reflection in a packaged EO modulator, however, major ones are caused by three positions, i.e. the RF termination, interfaces between input connector-to-input board and input board-to-modulator chip.
The main function of the RF termination is to electrically match the impedance of the EO modulator electrode throughout the frequency band of operation to ensure minimum RF reflection at the EO modulator input port. The input connector is the packaged EO modulator's interface to the outside world. The input board is the interface between the modulator chip and input connector. The RF termination absorbs the remaining RF power with minimum perturbation to the operation of the EO modulator.
If the impedance of the RF termination perfectly matches the impedance of the EO modulator electrode throughout the operating frequency band, there will be no reflection of RF power back to the RF input port. Consequently, an adequate impedance characteristic over the operation frequency band is probably the most important feature of the RF termination. To serve as a good RF termination for EO modulator applications, other important features have to be considered such as RF power handling (or dissipation) capability, temperature stability, ease of manufacturability, small size, low cost and low parasite parameters, etc.
An electrical signal typically in the RF band is fed into the input connector. The input board provides an electrical transition between the input connector and EO modulator chip, where the optical wave propagating in the optical waveguide is modulated by the electrical or RF wave propagating in the electrodes through the Electro-Optical effect. The unused or remaining electrical or RF power is dumped into the RF terminal in the end of the electrode.
With reference to FIG. 1, an example of a simplified prior art optical communication system 100 is shown, utilizing an EO modulator 107 of the present invention. The optical communication system 100 comprises a transmitter 110, a receiver 109 and a transmission medium 108, which connects the transmitter 110 to the receiver 109. The transmission medium 108 is typically an optical fiber.
The transmitter 110 includes a laser 104, which operates in accordance with laser control signals received from a laser controller 103.
A lensed optical fiber, or fiber pigtail, 113 receives the optical signals 112. The lensed optical fiber 113 is coupled to the isolator 105, which reduces optical reflections directed towards the laser 104. In one embodiment, the optical isolator 105 is combined with a polarizer (not shown) to further reduce reflections to the laser 104. In another embodiment, the lensed optical fiber 113 is coupled directly to the EO modulator 107, rather than through the isolator 105.
The EO modulator 107 receives the optical signals 112 from the laser 104 via an input fiber 106. The EO modulator 107 includes two waveguides 114 and 115. The controller 102 controls each waveguide 114, 115 independently of the other or with one control signal. The optical signals 112 are received at an input 116 of the EO modulator 107 and are modulated in each of the optical waveguides 114 and 115. Modulated optical signals from each of the optical waveguides 114 and 115 are combined into a modulated optical signal at an output 117 of the EO modulator 107. The EO modulator 107 may perform either amplitude modulation or phase modulation or some combination to “chirp” the light of the received optical signals 112. The combined, modulated optical signal is transmitted across the fiber 108 to the receiver 109.
The controller 102 receives digital data signals from a data source 101 via a transmission line 118, and generates modulation control signals in response to the received signals. The modulation control signals are introduced into the EO modulator 107 via leads 119 and 120. The modulation control signals are indicative of a predetermined modulation of the optical signals 112 and of desired modulation chirp parameters. For example, the modulation control signals are received by the EO modulator 107, and in response, the relative propagation velocities of each of the waveguides 114 and 115 changes to generate a desired modulation chirp parameter value. A single control signal may interact asymmetrically with waveguides 114 and 115 to produce a fixed amount of chirp.
The controller 102 also introduces a bias signal via lead 121 to the EO modulator 107 which sets its operating point. The bias signal may be either preset or generated in response to changing environmental conditions such as temperature, bias drift or charge accumulation in the vicinity of the electro-optic waveguides.
The prior art system described above could advantageously incorporate a RF termination according to the invention disclosed herein within the EO modulator 107, from which an additional RF output monitoring line 122 would connect to RF detection or monitoring circuitry 123.
FIG. 2 illustrates a top planar view of a prior art packaged EO Mach-Zehnder modulator of the optical communication system 100 of FIG. 1. A fiber optic cable 206 is in optical communication with an optical input 216 of modulator chip 207. The fiber optic cable 206 presents an optical signal from a light source or laser (not shown) to the input 216. The optical signal is split into two equal signals by a first optical Y-connection 225. RF electrodes 226 and 227 form an electrical transmission line for transmitting RF signals from an electrical input port 201 to an electrical output port 202. The RF signals are supplied by an external source through RF interconnect board 228 connected to the electrical input port 201. As the split optical signals propagate along optical waveguides 229 and 230, they are modulated by the electrical field of the RF signal. The distance in which the RF signals interact with, or modulate, the split optical signals is known as the interaction distance, and is determined primarily by the modulator design.
There are mainly two types of Lithium Niobate (LiNOb3)EO Mach-Zehnder (MZ) modulators used, X-cut and Z-cut types. FIG. 2 shows only the X-cut type.
A second optical Y-connection 231 combines the two split optical signals into a single, modulated optical signal. A fiber optic cable 208 which is coupled to an optical output 217 of the modulator chip 207, presents the combined optical signal to subsequent stages (not shown) of an optical communication system.
The modulator chip 207 includes a substrate 234 which in one embodiment is made of X-cut lithium niobate (LiNbO3) and is approximately 1000 microns (atm) thick. In another embodiment, the modulator chip 207 is made of Z-cut LiNbO3.
The optical waveguides 229 and 230 may be created by diffusing titanium into the substrate 234. In one embodiment, the optical waveguides 229 and 230 are formed by creating a strip or channel (not shown) in the substrate 234, depositing titanium in the channel, and then raising its temperature so that the titanium diffuses into the substrate 234. The optical waveguides 229 and 230 are approximately seven microns wide and approximately three microns deep.
Summarizing, the prior art RF termination board 235 is located at the electrical output port 202 of the electrodes 226 and 227 to absorb any unused or residual RF power.
A RF termination board 235 according to the invention disclosed herein would also include an integrated RF monitoring output port 236 for advantageously providing RF monitor signal for use in detection, monitoring or feedback circuitry in a compact and easily manufactured manner.
Commonly used RF terminations for EO modulator applications are lumped element and thick or thin-film resistors. Lumped element resistors used for EO modulators are typically surface-mounted ones, as shown in FIG. 3. A thick or thin-film resistor as the RF termination, as shown in FIG. 4, is made by hybrid circuit technology.
FIGS. 3a and 3b show a top planar view and cross-section along line A-A′ respectively, of a prior art example of a RF termination board 335 corresponding to the RF termination board 235 in FIG. 2. The RF termination board 335 is commonly formed on a ceramic substrate 236. Other materials with similar physical and electrical properties could also be used. A short section of RF transmission line 337, either a coplanar waveguide (CPW) or microstrip line, extends from the termination input port 302 at the edge of the RF termination board 335 to RF termination 339. The RF termination 339 can be either a resistor or complex circuitry of resistive and reactive passive components. A surface-mounted resistor in the form of a lumped element is shown in this example. The ground electrode 338 is connected through the vias 340 to the electrical ground plate 341. The electrical connection between the electrical output port 202 of the modulator chip 207 of FIG. 2 and the termination input port 302 is typically by gold wire bonding.
The main shortcoming of the lumped element resistor is its need for extra soldering process and the possibility of high parasitic microwave parameters.
FIGS. 4a and 4b are a top view and cross-section respectively of an alternative form of the RF termination board 235 in FIG. 2. This RF termination board 435 has similar components as the board shown in FIG. 3a except for the RF termination 339. The RF termination 439 is a thin- or thick-film resistor connected between RF transmission line 437 and ground electrode 438. The RF termination can also be formed by complex circuitry of resistive and reactive thin- or thick-film components. The ground electrode 438 is connected through the vias 440 to the electrical ground plate 441. The electrical connection between the electrical output port 202 of the modulator chip 207 of FIG. 2 and the termination input port 402 is typically by gold wire bonding.
A thick or thin-film resistor as the RF termination, such as shown in FIG. 4, can made by hybrid circuit technology, which is much lower cost and easier manufacturability for high-volume production, better repeatability, and smaller size compared to the lumped-element resistors. However, it is difficult to obtain a good impedance match over an appreciable electrical bandwidth.
The shape of the thick- or thin-film RF termination critically affects its frequency characteristics and inherent parasitics. An RF termination is not only an RF power absorber but also a transition section between the EO modulator electrode and the electrical ground. Various geometrical shapes have desirable affects on the terminating impedance match, especially when the length and size of the termination component is big enough to impact the RF impedance at higher frequencies. A tapered shape can be a desired shape for the transition of electromagnetic wave. In the example of FIG. 5 a taper is used in the design of electrical/RF transition between two transmission lines with quite different electrical/RF characteristics.
FIG. 5 shows top view of another prior art form of the RF termination board 435 in FIG. 4a. RF transmission line 537 extends from termination input port 502 to RF termination 539, which is a thin- or thick-film resistor with distributed resistance. The RF termination 439 is tapered from a narrow end at an end of the RF transmission line 437 to a wider end at its connection with ground electrode 438. The tapered sides of the trapezoid-shaped RF termination 439 can be described by linear, quadratic, exponential and any other gradual varying curves to ensure smooth dissipation of RF power with minimum RF reflection.
Typical configurations of a front-end RF signal detection scheme for EO modulators in optical fiber communication systems include a commercially available RF power splitter/coupler located in front of the input connector to the EO modulator. In this application, the coupler usually has high coupling ratio, that is, a high percentage of RF power goes through the main channel to the EO modulator, while only a small percentage of RF power, such as 1%, is coupled to the coupling channel for monitoring the RF signal. The small portion of RF power picked up by the coupling channel goes through an RF connector on the RF splitter/coupler, and fed to the input port of RF signal detection or monitoring circuitry. Then the tapped RF signal is detected by a (single) or two (balanced) RF diodes. The detected signal can be used as input signal for a RF signal detection circuit.
The main shortcomings of such an arrangement is the extra RF power loss and bulky size. Some RF power is lost due to tapping before the input signal is passed to the EO modulator. Additional coupler/splitters need more space and also make the manufacturing cost higher.
A configuration that attempts to address the power loss is a back-end RF signal detection scheme for EO modulators in optical fiber communication systems. A RF power splitter/coupler is located at the output port of an EO modulator. In this application, the coupler usually also has a high coupling ratio, that is, a high percentage of RF power goes through the main channel, with only small percentage of RF power, such as 1%, being coupled to the coupling channel. When the unused RF signal from the EO modulator is fed into the input terminal of the RF splitter/coupler, the majority of RF power goes through the main channel into the bulk RF resistor termination, typically 50 ohm. The small portion of RF power picked up by the coupling channel is fed to the input port of RF signal detection or monitoring circuitry, where it can be detected by a (single) or two (balanced) RF diodes.
The RF signal detection circuitry, including the RF diodes and passive components, can also be integrated into the RF termination board on a ceramic substrate using the hybrid PCB technology.
In the above examples, monitoring the RF conditions near the EO modulator involves bulky and possibly expensive components such as an RF power splitter/coupler and attendant connectors, which in themselves can be sources of parasitic impedances and reflections.
An object of instant application is to provide a termination with an RF tap for monitoring integrated on the same chip so that improved performance over a broader modulation frequency range can be achieved.